The present invention relates to monitoring techniques using magnetic circuits in which relative movement is produced between a magnetic circuit and at least one element with a peripheral irregularity and the passage of the peripheral irregularity near the magnetic circuit produces a variation in relunctance in the magnetic circuit which is detectably externally.
Such monitoring techniques are used, for example, in so-called phonic wheels, which are employed to an ever increasing extent in revolution counters, tachometers, etc. A particularly important field of possible use is the automotive sector in which phonic-wheel sensors can be used to detect the rate of rotation of the engine and/or of the wheels in association with electronic ignition and injection systems, automatic gear changes, anti-lock braking systems (ABS). . .
According to a current solution, to which FIG. 1 relates, a monitoring device indicated 1, with a magnetic circuit, is arranged in a position facing the edge of a disc D keyed on to a shaft W whereof it is wished to detect one or more parameters of angular movement, such as the position, the speed, the acceleration. For this purpose, the disc D is provided around its periphery with one or more irregularities consisting, for example, of a notch T, a hole, etc. The disc D is made entirely, or at least in the region adjacent the peripheral irregularity T, from a material which has a certain degree of magnetic permeability, typically a mild ferromagnetic material.
When the irregularity T passes in front of the sensor 1 as a result of the rotation of the disc D, it causes the reluctance in the magnetic circuit of the sensor 1 to vary. This variation in reluctance can be detected so as to cause the emission of an output signal, generally consisting of a train of pulses, on an output line 2. Each pulse indicates the transit of the peripheral irregularity T in front of the sensor 1. As the geonetrical parameters of the disc D are known, it is possible to deduce the angular position, the speed and the angular acceleration of the shaft W from the position of the pulses and their frequency of repetition.
The graph of FIG. 2 shows, by way of example, a typical time trace of the output signal V.sub.out detectable at the output 2 of an electromagnetic sensor of known type.
This signal is indicative of the temporal variation .delta..phi./.delta.t in the flux .phi. which flows through the electromagnetic detector. This flux varies periodically, as stated, due to the periodic variation in reluctance induced by the transit of the peripheral irregularity T in front of the sensor 1.
The signal trace indicated V.sub.1 in FIG. 2 is more than anything, a theoretical model. In reality, phenomena such as production tolerances in the disc D, its assembly in a slightly eccentric position relative to the shaft W, permanent deformations of the disc D or temporary deformations induced by vibrations, or non-homogeneity of the magnetic characteristics of the material constituting the disc disturb the output signal of the sensor 1.
Generally the aforementioned phenomena give rise to variations in the width of the air gap formed by the irregularity T during its passage in front of the sensor 1 or to anomalous fluctuations in the magnetic flux in the detector. Consequently, the reluctance of the magnetic circuit of the detector is modulated correspondingly and this is converted into a modulation (for example of sinusoidal type, as in the example illustrated by a broken line and indicated V'.sub.1 in FIG. 2) of the output signal detectable on the line 2. This modualtion makes it more difficult to detect the position of the signal pulses correctly. Errors can therefore be produced in the determination of the position of the pulses, which are expressed--in some cases very seriously due to propagation--as corresponding operational errors in the system in which the sensor is installed.
In order to avoid such drawbacks so-called differential sensors have been developed.
In such sensors, which are known, for instance, from FR-A-2 112 298 and FR-A-2 386 039 (corresponding to GB-A-1 302 507 and GB-A-1 595 680respectively), two adjacent magnetic circuits are formed with charactertistics of symmetry in at least one region and relative movement between the two magnetic circuits and the element with the at least one peripheral irregularity (i.e. the phonic wheel) is then caused so that the peripheral irregularity passes near the two magnetic circuits in sequence causing corresponding variations in reluctance in the magnetic circuits which can cause in the characteristics of symmetry in the at least one region a disturbance which can be detected externally thus monitoring the speed of said relative movement.
A basic drawback of the sensors of the prior art is that they are generally adapted for operating in conjuction with a moving element (i.e. a phonic wheel) having a substantially continuous plurality of irregularities (notches, holes, etc.) and cannot be used in a reliable manner together with a moving element having just one or several, spaced apart irregularities such as is the case of monitoring assemblies used for monitoring the angular position or phase of a rotating shaft. Moreover in certain arrangements such as disclosed in FR-A-2 112 298 a precise relationship must exist between the dimensions of the sensor and the dimensions of the irregularities of the phonic wheel. This means that one kind of sensor can be used in conjunction with one type of phonic wheel only, which may be quite unpractical in a number of applications such as those in the automotive field.